1. Field of the Invention
The present invention relates to a method of recycling a nickel-hydrogen secondary battery.
2. Description of Related Art
The nickel-hydrogen secondary battery has features of a great energy density and high energy efficiency. The nickel-hydrogen secondary battery also has features that, since the reaction therein is a simple reaction of hydrogen ions moving between active materials upon charging and discharging thereof, the battery can be sealed by controlling the charging and discharging conditions, and a sufficiently long cycle life can be obtained.
In this nickel-hydrogen secondary battery, nickel hydroxide is used as the active material of a positive electrode while a negative electrode is made of a hydrogen-occuludable alloy capable of occuluding hydrogen as the active material of the negative electrode. In addition, to improve the utilization rate of the active material of the positive electrode, cobalt oxide is also included. And an aqueous alkali solution is used as the electrolyte. With this battery, charging and discharging are performed by the following reactions.
positive electrode: EQU .beta.-Ni(OH).sub.2 +OH.sup.- .fwdarw..beta.-NiOOH+H.sub.2 O+e.sup.- (charging) PA1 negative electrode: EQU M+H.sub.2 O+e.sup.- .fwdarw.MH+OH.sup.- (discharging)
The separator which separates the positive electrode and negative electrode from each other is made of polypropylene or the like which is stable in alkali. Polypropylene is, however, low in hydrophilic property, so that the battery performance cannot be sufficiently achieved. To overcome this problem, sulfone groups as hydrophilic groups are chemically fixed to the surface of polypropylene fibers so that the separator of polypropylene nonwoven fabric exhibits high hydrophilic property and is stable in alkali.
The nickel-hydrogen secondary battery has excellent features such as being maintenance-free because of it's sealed arrangement, and long life times. However, it has become clear that where the nickel-hydrogen secondary battery is used in severe conditions such as high temperature atmosphere, or over-charging or over-discharging with a high electric current, deterioration as follows occurs. In the positive electrode, for example, the above normal charging and discharging cycle is not performed, and K.sup.+ ions, Na.sup.+ ions and the like are captured by the positive electrode to generate .gamma.-NiOOH. Consequently, the capacity of the positive electrode is reduced. In the negative electrode, MmNi.sub.5-x-y-z Al.sub.x Mn.sub.y Co.sub.z (Mm:misch metal) or the like as the hydrogen-occuludable alloy dissolves to generate Mm(OH).sub.3, and Al, Mn and Co also dissolve and then deposit as Al(OH).sub.3, Mn(OH).sub.2 and CO(OH).sub.2 on the surface of the negative electrode, which results in the surface of the negative electrode becoming inert. These deposits are also captured by crystals of the positive electrode active material which lowers the capacity thereof.
Furthermore, Mn ions resulting from the deterioration of the negative electrode, and oxygen radicals resulting from the battery reaction may act on the separator to break chains in molecules to drop the hydrophilic groups therefrom. In this case, the hydrophilic property of the separator decreases with the result that battery performance decreases.
Due to these reactions, the electrolyte is fixed (dried up), which makes charging and discharging difficult. ("fixing" indicates the phenomena such as change of OH.sup.- ions in the electrolyte into Mm(OH).sub.3, or capture of K.sup.+ ions by the positive electrode.
Conventionally, there is no method by which nickel-hydrogen secondary batteries which have deteriorated can be recycled so that they have been discarded, or disassembled to recycle parts or materials.
Cobalt which is used in the nickel-hydrogen secondary battery, is short as natural resources, and misch metal (Mm) used in the negative electrode is expensive. Consequently, it is very desirable to reuse nickel-hydrogen secondary batteries, which has deteriorated, by regenerating them so that they are capable of charging and discharging properly.